DNA in Forensic Contexts

With the exception of monozygotic (identical) twins, the genetic information of a person is unique and can, therefore, be used for human identification in conflicts, mass disasters or crime scene investigations.

As DNA is inherited from the parents, it can also be used to establish familial links. For example, in paternity, maternity or siblingship testing.

When using DNA to establish the identity of a body, it is essential that a biological sample or genetic profile of the missing person is available for direct comparison (see next step for more on this topic).

The basics of how DNA matching is done

Not all of our DNA is different, in fact, most of our DNA (approximately 99%) is identical to other human beings.

The simplest level of variability within the DNA is the change of single nucleotides, either in the form of mutations or Single Nucleotide Polymorphisms (SNPs). These changes can consist of:

• a replacement of one nucleotide by another (substitution)
• an addition (insertion) of one nucleotide
• a loss (deletion) of one nucleotide

A different level of variability can be found in the number of repetitions of a particular short DNA sequence, normally 2-5bp (base-pair) long. This type of polymorphism can be identified as Microsatellite, Length Polymorphism or Short Tandem Repeats (STRs), as the variability consists of the number of repeats present in each DNA sequence. The location in the genome of a STR or a SNP is usually referred to as ‘locus’ (singular) or ‘loci’ (plural).

Types of DNA polymorphisms. Copyright Eva Fernández-Domínguez.

For human genetic identification, a combination of highly variable STR regions dispersed across the genome is used. The number and location of the regions used varies depending on the country. The US uses a combination of 20 STR loci plus the Amelogenin gene used for sex determination known as the ‘CODIS Core STR Loci’. The UK uses a combination of 16 core STR loci plus Amelogenin known as the ‘DNA17 system’. Most European countries use the ‘European Standard Set (ESS)’, consisting of different 12 STR loci.

The combined information of these STRs (STR profile) provides a unique signature or ‘DNA fingerprint’, which is only shared between identical twins. Therefore, if the full STR DNA profiles of two samples are identical or ‘match’, it can be concluded that these two samples have the same origin or, in other words, that they belong to the same person. This can be useful in different forensic scenarios. In crime scene investigations, a biological sample of unknown origin found at the crime scene is genotyped and directly compared against reference samples, which could be biological samples taken from suspects or genetic profiles stored in a database of offenders, for example.

Human identification using DNA in a crime scene investigation. Copyright: Eva Fernández-Domínguez.

In the case of missing persons, when an unidentified corpse is found, a biological sample from the corpse is taken and a genetic profile recovered. This is then compared against:

1. a biological sample from the person (for example, cells in a toothbrush or biopsy specimens stored at the hospital)

2. or, more frequently- possible family members

In the case of possible family members, we will not be looking for a match but for a compatible pattern of inheritance, which will vary depending on the available family members and the type of DNA studied (nDNA, Y Chromosome or mtDNA).

In the example below individuals 1 and 7 are both males and are expected to share the same Y chromosome. If autosomal STRs are used instead, individual 7 will share 1/4 of his genetic information with individual 1, so the genetic profile of all the individuals in the pedigree will be needed to achieve a meaningful comparison.

Human identification using DNA in a case of missing persons. In the family pedigree, squares represent male and circles female individuals. Copyright: Eva Fernández-Domínguez.

Next find out how DNA is sampled from different bones!